1. Field of the Invention
The present invention relates to preparation of injectable compositions. More particularly, the present invention relates to injectable suspensions having improved injectability, and to methods for the preparation of such injectable suspensions.
2. Related Art
Injectable suspensions are heterogeneous systems that typically consist of a solid phase dispersed in a liquid phase, the liquid phase being aqueous or nonaqueous. To be effective and pharmaceutically acceptable, injectable suspensions should preferably be: sterile; stable; resuspendable; syringeable; injectable; isotonic; and nonirritating. The foregoing characteristics result in manufacturing, storage, and usage requirements that make injectable suspensions one of the most difficult dosage forms to develop.
Injectable suspensions are parenteral compositions in that they are introduced into an organism or host by means other than through the gastrointestinal tract. Particularly, injectable suspensions are introduced into a host by subcutaneous (SC) or intramuscular (IM) injection. Injectable suspensions may be formulated as a ready-to-use injection or require a reconstitution step prior to use. Injectable suspensions typically contain between 0.5% and 5.0% solids, with a particle size of less than 5 μm for IM or SC administration. Parenteral suspensions are frequently administered through needles about one-half to two inches long, 19 to 22 gauge, with an internal diameter in the range of 700 to 400 microns, respectively.
To develop an effective and pharmaceutically acceptable injectable suspension, a number of characteristics must be evaluated. These characteristics include syringeability, injectability, clogging, resuspendability, and viscosity. As will be readily apparent to one skilled in the art, other characteristics and factors should be considered in developing an injectable suspension (see, for example, Floyd, A. G. and Jain, S., Injectable Emulsions and Suspensions, Chapter 7 in Pharmaceutical Dosage Forms: Disperse Systems Vol. 2, Edited by Lieberman, H. A., Rieger, M. M., and Banker, G. S., Marcel Dekker, New York (1996), the entirety of which is incorporated herein by reference and referred to herein as “the Floyd et al. Chapter”).
Syringeability describes the ability of an injectable suspension to pass easily through a hypodermic needle on transfer from a vial prior to injection. It includes characteristics such as ease of withdrawal, clogging and foaming tendencies, and accuracy of dose measurements. As described in the Floyd et al. Chapter, increase in the viscosity, density, particle size, and concentration of solids in suspension hinders the syringeability of suspensions.
Injectability refers to the performance of the suspension during injection. Injectability includes factors such as pressure or force required for injection, evenness of flow, aspiration qualities, and freedom from clogging.
Clogging refers to the blockage of syringe needles while administering a suspension. It may occur because of a single large particle, or an aggregate that blocks the lumen of the needle due to a bridging effect of the particles. Clogging at or near the needle end may be caused by restrictions to flow from the suspension. This may involve a number of factors, such as the injection vehicle, wetting of particles, particle size and distribution, particle shape, viscosity, and flow characteristics of the suspension.
Resuspendability describes the ability of the suspension to uniformly disperse with minimal shaking after it has stood for some time. Resuspendability can be a problem for suspensions that undergo “caking” upon standing due to settling of the deflocculated particles. “Caking” refers to a process by which the particles undergo growth and fusion to form a nondispersible mass of material.
Viscosity describes the resistance that a liquid system offers to flow when it is subjected to an applied shear stress. A more viscous system requires greater force or stress to make it flow at the same rate as a less viscous system. A liquid system will exhibit either Newtonian or non-Newtonian flow based on a linear or a non-linear increase, respectively, in the rate of shear with the shearing stress. Structured vehicles used in suspensions exhibit non-Newtonian flow and are typically plastic, pseudoplastic, or shear-thinning with some thixotropy (exhibiting a decrease in viscosity with an increase in the rate of shear).
In design of injection vehicles, viscosity enhancers are added in order to retard settling of the particles in the vial and syringe. However, viscosity is typically kept low, in order to facilitate mixing, resuspension of the particles with the vehicle, and to make the suspension easier to inject (i.e., low force on the syringe plunger). For example, Lupron Depot from TAP Pharmaceuticals (mean particle size of approximately 8 μm) utilizes an injection vehicle with a viscosity of approximately 5.4 cp. The fluid phase of a suspension of Decapeptyl from DebioPharm (mean particle size of approximately 40 μm), when prepared as directed, has a viscosity of approximately 19.7 cp. Conventional parenteral suspensions are dilute, with limitations for viscosity because of syringeability and injectability constraints. See, for example, the Floyd, et al. Chapter noted above.
Injectable compositions containing microparticle preparations are particularly susceptible to injectability problems. Microparticle suspensions may contain 10-15% solids, as compared with 0.5-5% solids in other types of injectable suspensions. Microparticles, particularly controlled release microparticles containing an active agent or other type of substance to be released, range in size up to about 250 μm, as compared with a particle size of less than 5 μm recommended for IM or SC administration. The higher concentration of solids, as well as the larger solid particle size, make it more difficult to successfully inject microparticle suspensions. This is particularly true since it is also desired to inject the microparticle suspensions using as small a needle as possible to minimize patient discomfort.
Thus, there is a need in the art for an injectable composition with improved injectability. There is a particular need in the art for an injectable composition that solves the injectability problems associated with microparticle suspensions. The present invention, the description of which is fully set forth below, solves the need in the art for such injectable compositions.